1. Field of the Invention
Apparatuses and methods consistent with the present invention relate to operating a screen in a water or wastewater stream based on calculated screen blockage percentages and the velocity throughput of the screen.
2. Description of the Related Art
Today, with the increased sophistication of processes and treatment, screening devices also need to be more sophisticated, reliable and efficient. Because of the increased level of sophistication of the entire process, the screening devices are relied upon to capture increasingly higher levels of debris from the waste stream.
The move to finer screening has enabled improved downstream processes of a wastewater and water treatment plants, lower levels of solids in storm and combined sewerage outfall, and lower levels of solids in water inlets for cooling and processing. Finer screening results in a headloss increase through the screening device. This headloss is due to the relative open area of the screening device and blockage by debris as the waster or wastewater flows through the device.
Various types of screens are used in a variety of applications including Wastewater Treatment, Industrial Water and Wastewater Treatment, Fresh and Salt Water Intake, Storm Sewage Outfall, Combined Sewage Outfall, Pump Stations, and others. In particular, mechanical screens are designed to collect and remove foreign debris that exists in the influent or effluent flow of various processes.
Screen types can consist of screening media constructed of perforated panels, slotted panels, bars, wire mesh and others. The media is most typically constructed of steel, stainless steel, plastic (UHMW-PE), and others. The screens can be cleaned with spray wash water (cold and/or hot), rotating brushes, stationary brushes, wiper blades, or any combination of these. The screen device can be placed in an open channel, in-tank/vessel installation, in front of a water supply pipeline as well as other installed configurations. Headloss is the measured difference in water level when comparing the downstream side to the upstream side of a Screen Device. Depending on the screening media, each type of Screen Device (i.e. Bar Screen, Drum Screen, Perforated Plate Screen, Mesh Screen, Wedge Wire Screen, Disc Screen, Etc.) has a fixed open area for the influent flow to pass through, relative to the upstream water level. The available open area for flow to pass through the Screen Device is greater when clear of debris. As debris collects and builds on the Screen Device during operation, the available open area for flow to pass diminishes.
In the related art, screens are typically operated using a level sensing system using level sensors. Often a PLC in the screen controller is programmed to calculate the differential between the upstream and downstream water levels to determine when to run the screen. The related art relies on a simple measurement of upstream water level as compared to downstream water level (Differential) to determine when the screen should be activated to clean the debris from it. Often times, this method is tied into a timer to periodically operate the device when differential does not initiate operation within a set time period. These simple systems only signal the need for the screen device to start and continue running but do not continuously calculate the change in blocked area resulting in inefficient and unnecessary running of the screen device. This periodic operation is often based on a predetermined value that is typically chosen after testing the screen under various flow conditions at the site. Flow rate and solids content is widely variable through the course of time rendering the simple method of level differential an inefficient way to determine the need to clean debris from the screening device.
Notably, the related art fails to take into account the velocity of the fluid flow and its impact on screening. As a result, the screening system may allow an excessive solid bypass as larger solids can be forced through the screen openings due to high velocity in view of the screen being cleaned too frequently.